1. Field of the Invention
The invention pertains to a method for an imaging ultrasonic inspection of a three-dimensional workpiece, in which ultrasonic waves are coupled into the workpiece with one ultrasonic transducer or a number of ultrasonic transducers and ultrasonic waves reflected within the workpiece are received by a number of ultrasonic transducers and converted into ultrasonic signals forming the basis of the non-destructive imaging ultrasonic inspection.
2. Description of the Art
The non-destructive inspection of a test body by means of ultrasound, for example, for testing materials for material defects such as cracks, inclusions or other material nonhomogeneities comprises the coupling of ultrasonic waves into the test body, the detection of the ultrasonic waves that are transmitted through the test body or reflected, diffracted, scattered and/or refracted within the test body, as well as the evaluation of the ultrasonic waves that are converted into ultrasonic signals.
The above-described known inspection method makes it possible to determine and evaluate the ultrasonic transmission properties and reflection properties of a test body. In this method that was originally developed in medical engineering (ultrasound diagnostics), defective spots such as material cracks, foreign inclusions or material boundaries situated within a test body can be illustrated in the form of regions with different reflection properties by evaluating the received ultrasonic signals accordingly. The position, shape and size of the defective spots can be illustrated three-dimensionally with a high resolution.
It is quite obvious that this method is used in numerous fields of application. For example, this method is used for testing and measuring homogeneity or stability properties of structural components (concrete walls, ceiling or wall elements, etc.) or for detecting cracks, for example, in wheels of rail vehicles or aircraft parts.
A number of ultrasonic transducers that are combined into a so-called ultrasonic probe or array probe in order to simplify their handling are used in many instances of non-destructive material testing by means of ultrasound. There basically exist two types of ultrasonic probes. A pulse-echo probe is used if the probe couples an ultrasonic wave packet into the test body and then receives the ultrasonic waves reflected within the test body. Probes with separate ultrasonic transducers for coupling in and for receiving ultrasonic waves are referred to as transmitting-receiving probes.
In all ultrasonic test systems with sound field control known to date, the individual ultrasonic transducers are acted upon in a time-coordinated fashion such that the ultrasonic transducers can be activated independently of one another in a time-controlled fashion and, for example, serve as ultrasonic transmitters or receivers. Such a separate activation makes it possible, in particular, to respectively operate the individual ultrasonic transducers with different phase position and amplitude.
Array probes, in which a number of individual ultrasonic transducers are provided in order to realize a controlled emission and detection of ultrasonic waves, are able to stimulate ultrasonic waves in a test body under arbitrary acoustic irradiation angles and in any predefined focusing ranges and to receive ultrasonic waves from these acoustic irradiation regions. In order to carry out such a measurement for investigating the ultrasonic transmission capability of a test body, a control device acts upon at least one ultrasonic transducer of the array probe, usually several ultrasonic transducers of the array probe for a limited, brief time interval in order to couple ultrasonic waves into the test body. The thusly created ultrasonic wave packets being coupled in are reflected, for example, on material discontinuities within the test body and returned to the ultrasonic transducers that now operate as receivers in the form of reflected ultrasonic waves, wherein the ultrasonic transducers convert the reflected ultrasonic waves into ultrasonic signals and transmit these ultrasonic signals to the control device for evaluation purposes. The time period between the beginning of the transmission process and the end of the reception process of the ultrasonic signals is usually referred to as a measuring cycle.
In many instances, it is important to determine the transmission and reflection properties of a test body with the highest possible resolution within the test body volume. To this end, the time delays for the transmission cycles are adjusted accordingly in order to predefine the acoustic irradiation direction as well as the depth of field. The received ultrasonic signals of the individual ultrasonic transducers of the array probe are added up with corresponding phase delays such that an ultrasonic signal for a certain acoustic irradiations angle and, if applicable, a certain depth of field is generated in a transmission cycle, wherein such instances are referred to as so-called A-images. The A-image represents the ultrasonic echo along a predefined “viewing or sound propagation direction” through the test body. Such an image can be considered as a 1-dimensional sectional image similar to a line of section through the test body, along which ultrasonic echo signals are illustrated in a spatially resolved fashion. If the ultrasonic transmission through the test body takes place at different angles, that is if the sound beam is pivoted in the test body, preferably within a uniform pivoting plane, it is possible to reconstruct a so-called sector image that is composed of a number of individual A-images.
The disadvantages of utilizing the phased array method for a non-destructive material inspection on a test body, however, can be seen in the expenditure of time and the metrological expenditure until a test body is inspected in a largely complete fashion, namely because it is essential for a complete signal evaluation to obtain sufficiently reliable measuring signals from all regions of the test body volume. For example, only information on the reflection properties along a predefined acoustic irradiation direction of the test body can be obtained in one measuring cycle or a number of individual measuring cycles with identical phase control of the ultrasonic transducers. Consequently, an inspection of the entire test body volume requires a very large number of measurements with respectively different phase controls such that the expenditure of time for the complete material test is quite high. The adjustment of a new acoustic irradiation angle or a new focal position also requires a labor-intensive and time-consuming reprogramming process.
Another disadvantage can be seen in that a predefined acoustic irradiation angle specifies the probe aperture, that is, the aperture cannot be optimally chosen for all acoustic irradiation angles such that the resolution of the measurements deteriorates.
If controls with respect to the manufacturing quality of industrial products should be carried out online by means of currently available ultrasonic test techniques, that is, as an integral part of the manufacturing process, the measures required for the quality inspection should not have any effects that impede or slow down the manufacturing process. The manufacture of workpieces realized in the form of extruded profiles, for example, steel billets, rods or profiles of any type such as, in particular, rails manufactured in an extrusion moulding process requires reliably operating test methods in order to fulfill the strictest quality requirements. As initially mentioned, online inspections of workpieces in the form of extruded profiles that are transported or conveyed along production lines with speeds of a few meters per second by means of ultrasonic test techniques known so far are not sufficiently fast and associated with excessive costs and device expenditures. Even imaging reconstruction methods that are based on the so-called synthetic aperture technique (Synthetic Aperture Focusing Technique—SAFT), in which all ultrasonic signals received at different measuring points of the test object are projected back into the material, require a substantial expenditure of time for the measurement and the image reconstruction such that they are completely unsuitable for an online inspection. The reasons for this substantial expenditure of time can be seen in the recording of the ultrasonic signals with a moving ultrasonic transducer and the time-consuming evaluation of the recorded ultrasonic time signals for the image reconstruction.
Another disadvantage of thus far known ultrasonic test techniques that utilize array systems can be seen in the restricted mutual spacing between the ultrasonic transducers that should be smaller than half the wavelength of the ultrasonic waves to be coupled into the respective test body so as to prevent false echoes or artifacts in the reconstructed ultrasonic image.